So as you can see in the attached photo, I drew an equilateral triangle.
The first step was to write the measurements. In a equilateral triangle, the measurements are all the same. In this case 22 cm.
Next split the base in half. I did this with the green line. The base measurement is equal to 1/2 of 22 which is 11. This creates a 90° angle. (in red). Also in red I have marked the other angles. In an equilateral triangle all the interior angles are 60°. The top one is split in two.
Next, use the Pythagorean theorem. a² + b² = c².
In this problem C = 22 and b = 11.
a² + 11² = 22²
a² = 22² - 11²
a² = 484 - 121
a² = 363
a = √363
a = 19.05
The altitude of your triangle is 19.05 or √363 cm.
Hope that helps!